With his contract at Glory at an end, Colosimo was linked with a move to Sydney FC in 2008, receiving a hostile reception from Perth supporters[3] who let him know he was no longer welcome and was dropped from the starting side for some final matches.[4] Unable to reach a contract agreement, Perth released him from the club on 22 January 2008.[5]

It was announced by Sydney FC that Colosimo had signed a two-year deal, and he joined the club from the start of the 2008–09 season.[6] Colosimo became a standout performer as a centreback, with consistent performances impressing through the new 2009–10 season. On 31 March 2010, the defender switched clubs to Melbourne Heart, along with his teammates Clint Bolton and John Aloisi.[7] He became the captain of the new club and scored one goal in their debut season, an important equaliser against Adelaide in the final minutes of the game, which Heart triumphed 2–1 in stoppage time.

Colosimo first entered the Australian national selection framework at Under-20 level in 1997 while at the AIS. This led quickly to a national team debut in 1998 against Fiji at just 19 years of age. He remained in the national focus, selected as part of the squad at the 1999 FIFA World Youth Championship.

Later that year he was called up by then Socceroo manager Raul Blanco to make an appearance in the Socceroo's friendly against Manchester United on 18 July at the MCG. What started as a spirited game finished in horror for Simon, as Andy Cole lunged into a tackle with his leg raised and studs showing, felling the Australian midfielder in full-flight.[9] Simon was stretchered off in pain by stewards, and was diagnosed with having sustained tears to his anterior cruciate and medial ligaments, needing a complete knee reconstruction. He underwent surgery soon after, which included his anterior cruciate being replaced with a piece of his hamstring tendon. A rehabilitation process began, in which Simon would not kick a ball for six months. There was talk at the time of impending legal action against Cole by Carlton's General Manager Lou Sticca,[10] though no formal proceedings were ever entered into.

Following his recovery, he was selected for the 2000 Summer Olympics in Sydney, and was part of the national team squad for the 2005 Confederations Cup. Colosimo was selected in matches leading up to the Asian Cup 2007, but was not included in the squad for the final tournament although he was named as a standby reserve.